The perimeter of a rectangle is the total length of all the sides of the rectangle added together. To find the perimeter of a rectangle, we can use the following formula:
Perimeter = 2(length + width)
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In this case, the length of the rectangle is 21 feet and the width is 24 feet. Therefore, the perimeter of the classroom is:
Perimeter = 2(21 + 24) = 90 feet
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So the answer is 90